UIKit is the UI framework in the IPhoneRuntime, the equivalent of AppKit for traditional OS X apps. Most of the classes are more or less equivalent to their NS- prefixed AppKit counterparts.
Warning: The information listed here may be incomplete, incorrect, or extremely misleading. If in doubt, wait until the official SDK is released.
Useful classes:

• UIHardware: information about the device
• UIApplication: main application class
• UIWindow
• UIResponder
• UIView
• UIControl
• UIImage
• UIImageView
• UINavigationBar: That swank little bar at the top with a button or two
• UIButtonBar
• UITextView
• UITextLabel
• UITextField
• UIKeyboard: The iPhone's keyboard
• UIFontChooser
• UISegmentedControl
• UISliderControl
• UISwitchControl: On/Off switch
• UIPickerView
• UIPushButton
• UIAlertSheet
• UITransitionView
• UIProgressHUD
• UITable: iPhone's list UI. Related classes:
  
  • UITableColumn
  • UITableCell
  • UIImageAndTextTableCell
  • UIPreferencesTable
  • UIPreferencesTableCell
  • UISectionList
• UIAnimation